Article
Reference
The evolution of the northern margin of the Tethys in eastern Switzerland
FUNK, Hanspeter, et al.
Abstract
The former northern margin of Tethis is being studied in the Alpine chain by IGCP Project 198("Evolution of the Northern Margin of the Tethys"). This paper summarizes the present state of the knowlege from the opening phase of this Mesozoic to early Paleogene seaway in Eastern Switzerland. Basement rocks and sediments of the previous margin are now exposed here in the stack of the Alps and their European foreland.
FUNK, Hanspeter, et al . The evolution of the northern margin of the Tethys in eastern Switzerland. Episodes , 1987, vol. 10, no. 2, p. 102-106
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_ 犷 T h e E v o l u t i o n o f t h e
U N E S } U No r t h e r n Ma r g i n o f Te t h y s
i n East ern Swi t zerl and
by Ha ns pe t e r Funk, Ro l a nd Ob e r hd ns l i , Adr i a n Pf i f f ne r ,
St ef an Schmi d and Wal t e r Wi l di
The former northern margin of Tethys is being studied in the Alpine chain by IGCP Project 198 ("Evolution of the
Northern Margin of the Tethys"). This paper summarizes the present state of knowledge of the opening phase of this
Mesozoic to early Paleogene seaway in Eastern
Switzerland. Basement rocks and sediments of the previous margin are now exposed here in the nappe stack of the Alps and in their European foreland.
Introduction
The Alpine Tethys is the oceanic basin that opened by extensional and strike-slip tectonics from Triassic time onward, separating the former Pangaea into the Gondwana
continent in the south and Laurasia in the north. This
marine opening was synchronous with the beginning of the expansion of the North Atlantic basin. However, the Alpine Tethys closed again by orogenic inversion beginning near the end of Early Cretaceous time, some 110 Ma ago. Thus, the former continental margins of the Tethyan basin are now exposed on land, in the outcrops of the Alpine chain (Fig. 1).
Within the framework of IGCP Project 198, European geol- ogists are now seeking a better understanding of the evolu- tion of the northern margin of the Tethys from Triassic to early Paleogene times, which must obviously have been quite different from that of the southern margin. A key area for the northern margin is Eastern Switzerland, where the geological history is particularly well studied. Two main problems arise here: what is the most coherent palinspastic reconstruction of the former paleogeographic realm obtain- able by restoring the Alpine tectonic history; and which
mechanisms of basin subsidence predominated 一extensional tectonics, post-Hercynian thermal relaxation, or strike-slip tectonics.
The Mesozoic to early Paleogene margins and basins of Tethys in Eastern Switzerland are found in several geolog- ical units, (Fig. 1), which are reviewed from north to south in the following. Figure 2A is a synoptic cross section showing the relative position of the various units discussed
in the text.
On the southern margin of the Alpine Tethys (the Austro- alpine realm and Southern Alps of Figs. IB and IC), Meso-
zoic extensional tectonics resulted in tilted blocks and
basins, as outlined by Bernoulli and others (1979) and Winterer and Bosellini (1981). The style of this margin has been compared to the tectonic style of the Armorican con- tinental margin of Bay of Biscay, as revealed by reflection seismics (Bally et al., 1981).
The European Foreland
The European Foreland is represented by the fold-and-thrust belt of the Jura mountains, composed of Mesozoic sediments
and the former intracontinental Jura basin. The sediments
are mainly evaporitic and shallow-water Triassic carbonates
and Jurassic m arls and shallow-water lim estones. Below
this sedimentary cover, the meso-European crystalline base- ment is cut by an east-west, late Hercynian intracontmental
A Europe 日
rift, with continental elastics and coal seams.
! To the southeast is the M olasse
衅
诊 入
理一 -
? I Iberia F] G. I B
霭 } S E C T
匆2I O N S
BF I G 2 {
弓 口
Plateau, which constitutes
M iocene foreland basin of
crystalline basement here is
the the
of the Swiss
Oligocene-
Alps. The overlain by a sequence, which is
Africa
缈l l e l 多粼
o l l -
Messtaitthrustom ewhat tiltns.edozoiThe M olc sedihinner thanmenasse seditary im entn thes Jura m oun-are slightly
on the Plateau and form a foldand
OUTHERN belt
Alps (left photo).
at the external
side of Fig. 2A,
m argin and see
of the
c ov e r
N}C
100 knn
卜一-------一 ,
乏 1
眯 o Genova
国 口
FUROPEAN MARU NAL PLATFORM 八N D B八 、IN %
OCEANIC SOUTH-PENNINEC REAL .
NORTi卜 八N几 从]n pENNINIC NO仪TH卜胶N C ON IIN EN TALM 八R G IN UNHT }
八DRIA, SOL丁FERN CONTINENTAL MARGIN AND PL八117ORM
Figure 1: The regional setting and main geological features of the Alps.
二: Main trends of the Alpine chain in the Mediterranean realm; B:
Simplified geological map of the Western an d Central Alps; C:
Simplified palinspastic map of the Central and Western Alps in Late
Jurassic tim e.
102 EPISODES, Vol. 10, No. 2, June 1987
A
N S
户
/ 7me!_s
鳄
Iamb,10 10
20knn 二二二与 20km
no vertical exageration
B
1 ) N W
SESw iss
PLATEAU
AAR
MASSIF Vattis
HELVETIC PLATFORM
Alvier
NORTH一PENNINIC BASIN SOUTH-PENNINIC
a b s e n t ( s e d i m e n t a r y g a p . e r o s i o n ? ) __
下一 ,} ;x-- 共己三 今 百荟工
PR舫 TIGAU ADDLA AVERS
叁攀篡彗
L C}BASIN
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ow ret
争B u n d n e r s c h i e f o r
二~ .~~一 一‘二一一一升一 ‘ .. --~一 - .. -
Ophiolit,s Permian rift.c o义ntinental
clastics ii︶
译︒
一 羚
The Alps
The ancient Tethyan basin and continental margin units of Europe (the northern margin) and of Adria (the southern margin) constitute a pile of thrust sheets (Fig. 2A). Within these nappes, the Mesozoic sediments define three geolog} ical mega-units, namely the Helvetic, the Perminic and the Austroalpme zones. As demonstrated in the following, the northern mar-gin is preserved in the Helvetic zone and in the northern and middle units of the Penninic zone (Fig. 113 and C). The south-Penninic units are "oceanic" basin relicts, and the Austroalpine zone represents the Mesozoic southern margin of the Tethys, which belongs to the Adriatic prom-
ontory of the African continent (Laubscher, 1975, Bernoulli et al., 197 9, Derc ourt et al., 198 5).
In the Helvetic zone, the Hercynian basement of the Aar massif and its sliced-up Mesozoic and Tertiary cover are overlain by the Helvetic nappes (71-iimpy, 1969, Milnes and Pfiffner, 1977, Pfiffner, 1978, 1981). In all units, thin epi- continental sediments of Triassic age are followed by Juras- sic to Eocene platform units. Compression and thrusting in
this zone started in the Late Eocene and continued into
Middle Miocene time (Pfiffner, 1986). The north-Penninic units are composed
basement nappes (Gotthard, of Alpine crystalline Lucomagno,
Tambo and Suretta, see Fig. 2A)
sequences. The latter comprise
omagno, Simano, Adula,
and of thick sedimentary
m itic Lower
shaly,
Late
sedim ents Cretaceous
of Triassic age,epicontinental and dolo-
turbiditic Jurassic and
shales and arenites (BUndnersehiefer) and
arenitic and Cretaceous to
conglomeratic Early EocenE
The sedim ents are detached from
ment. Alpine metamo甲hism in
deep-water clasties of age (Pr}ttigau Flysch). their crystalline base- these units is mainly epizonal, with amphibolite facies in the extreme south. The mid-Penninic units consist of thick platform carbonates of Triassic to lowermost Cretaceous age in the north, and Triassic carbonates and Jurassic to Lower Cretaceous scarp breccias in the south. The Upper Cretaceous to Paleogene
deposits are hernipelagic and turbiditic.
EPISODES, Vol. 10, No. 2, June 1987 103 Figure 2: Paleogeographic evolution of the northern
margin of the Tethys in Eastern Switzerland along transects shown in Fig. 1B. A: Synoptic cross section, indicating the relative position of the various units discussed in the text. B: Paleogeo-
graphic section from the Jura basin to the Penninic basin the Barremian (Lower Cretaceous)。
一
01
2
E
3匕
4七
5名
6毛
7 任
8器
9
10
Figure 3: Geohistory diagrams of the continental margin units of Eastern Switzerland, showing total subsidence of pre-Mesozoic basement. (a) Jura basin of Eastern Switzerland。 (b) Southern part of the Helvetic platform (Alvier section). (c) Hypothesis for the subsidence of the north-Penninic Pr9ttigau basin. (d) Mid-Penninic Klippen unit in Central Switzerland. e) South-Alpine Generoso basin.
NW S
巍髦夔彝一
蘸 蘸馥酬夔辨{
SK蘸 鬓
U 】
轰器派
擎 哪龟扁饭
彝然Figure 4: The SNntis nappe, an allochthonous platform uni t in Eastern Switzerland。 Th e lithological units are laterally very continuous,
and synsedimentary extensional tectonics can only be deduced from precise一_measurements of sediment thickness. UJ: Upper Jurassic limestone formation.
SK: "Schrattenkalk," a platform formation of Barremian to early Aptian age.
sea-level changes and a somewhat irregular basin subsidence
controlled the sedim entation. This was eithpr of qhqllnw
basin type with deposition of marls,
form progradation (Bolliger and Burri,or dominated by plat- 1970, Gygi, 1986). In this basin, no extensional tectonic features such as listric faults or scarp breccias have been observed until now. We, therefore, postulate that subsidence was due to thermal relaxation after a late Hercynian thermal event. Creta- ceous sediments are absent due to erosion and/or a sedi- mentary gap.
It has been suggested that mid-Penninic units exist in two different structural situations (Fig. 2A; Streiff, 1962, TrUmpy et al., 1969). In the first, the Falknis-Sulz flub nappes overlie the north-Penninic units and are themselves overlain by south-Penninic ophiolites. In the second settinLy. ine zicnams nappes envelop the front of the Suretta base-
ment nappe and overlie the Avers Biindnerscbiefer containing basic voleanics in blueschist facies. However, as north-south and east-west nappe transport is suspected in this area, the debate is still open concerning the paleogeographic origin of the Schams nappes.
The Helvetic platform of eastern Switzerland existed from the Triassic onward, in
continental basin fill,
只 号 r t
tn e
on the top of a late
"Verrucand' trough Jurassic and Cretaceous sediment sequencesa re
Paleozoic
(Fig. 213).
dom inated
The south-Penninic units in this transect of the Alps are
s l i c e s o f 下丽币石孤歹妥蔽几n d o f t h e P l a t t a n a p p e wi t h o p h i -
olites, and pelagic and hemipelagic sediments of Jurassic and Cretaceous age. The Austroalpine zone comprises the Silvretta basement nappe and the Mesozoic cover sediments of the Northern Calcareous Alps (Fig. 2A). Deformation in the Penninic zone and at the southern margin of the Tethys( t h e Au s t r o a l p i n e r e a l m) s t a r t e d i n E a r l y C r e t a c e o u s t i 币e
and continued into the Paleogene.
by carbonates, grading into a more marly sequence in the southern part of the shelf. The total thickness of Mesozoic deposits is about 1.5 km in the north and 3 km in the south.
Despite the lack of synsedimentary faults and conglomer- ates, extensional tectonics appear to be mainly responsible for the subsidence, because the geohistory diagrams (Fig. 3)
indicate a stepwide subsidence with the characteristics of
e x t e n s i o n a l ma r g i n s ( F u n k , 1 9 8 5 ) . Mo r e o v e r , i s o p a c h ma p s
and precise palinspastic reconstructions of the sediment prisms illustrate sedimentation in tilted basins from theEarly Jurassic to early Barremian (TrUmpy, 1980, Strasser and Funk, 1977). No evidence of extensional tectonics has
been shown to exist from the Barremian (tOD of I'Schrat- i:enKalK,- rig. 4) to the early Faleogene when global sea- level changes and detrital input were controlling the
QPtiim PnfnH nn
Triassic to Low er Cretaceous Evolution
The whole area that subsided during the Mesozoic to form the northern margin of the Alpine Tethys was underlain by a Hercynian (Variscan) crystalline basement with east-west and northeast-southwest late Bercynian rifts. Since the opening of the marine basins only began in Triassic to Early Jurassic time, and since orogenic inversion of the basins started in the Early Cretaceous, the discussion here of the evolution of the margin is generally limited to this time
interval.
In Early Triassic to Late Jurassic times. the Jura basin krig. ztj) was a slowly subsicing trough together with the northern and central part of the Swiss Plateau. The south-
e r n p a r t o f t h e P l a t e a u a n d t h J蔺石歹万厂王 蔽 Aa r ma s s i f
emerged during early and middle Liassic (the Alemanic land of TrUmpy, 1949). In the Middle and Late Jurassic, global104 EPISODES, Vol. 10, No. 2, June 1987
A p a l e o g e o g r a p h i c g a p ( " mi s s i n g l i n k " o f F i g . 2 1 3 ) e x i s t s
between the southernmost outcrops of the Helvetics and the northernmost outcrops of the north-Penninic basin. The Late Cretaceous and early Paleogene Sardona Flysch, an exotic strip sheet within the Helvetic nappe stack, may represent a relict of this area (RUefli, 1959).
The north-Penninic PrMtigau basin began with epiconti- nental and shallow-marine deposits in the
Jurassic, and then subsided rapidly during iassic and Early Cretaceous, as indicated by the turb
Tr Jurass
iditicic and Early deep-water sedimentation of the BUndnerschiefer (Ngnny, 1948, Pantic and Isler, 1978). In the Valais basin, farther west, coarse
念篇; 黯e i n L t r i a a s b s a i s c i 群瞬 s 摆e 默 嚼豁篡n e
t h i n k t h a t s t r o n g , s u b s i d e n c e a n d 。 b a s e m e n t t o p o g r a p h y
i n d i c a t e a r e g i m e o f e x t e n s i o n a l t e c t o n i c s i n t h i s a r e a .
When taking into account the presence of helminthoid trace fossils in the Upper Cretaceous flysch sequence of the Prdttigau, one can estimate water depth at that time of several hundred metres. In this case, the total pre-orogenic subsidence of the basin is about 5 to 7 km, and the width after stretching is estimated at 80 to 100 km. Lateral sediment input from the west rather than from the east has to be postulated,for most of the Cretaceous time; Paleocene and Eocene sediment transport directions are from the north
t o t h e s o u t h , mo s t p r o b a b l y f r om a n o r o g e n i c s c a r p a t t h e
northern ma馆in, to the Priittigau basin in the South (Nanny,1948, Allemann, 1957).
sedimentary record. Late Cretaceous to early Paleogene sedimentation was controlled by global sea-level changes and terrigenous sediment input. Strong extensional tec- tonics with subsidence are indicated in the neighbouring basin to the south, the north-Penninic Priittigau basin, with turbiditic deep-water clastics from Jurassic to Early Eocene time (Biindnerschiefer and flysch sedimentation).
The interpretation of the transition from the extensional part of the European margin to the south-Penninic realm with oceanic crustal characteristicsis
hypothesis suggests high with platform
Penninic basin from
that a continuous
controversial. One east-west structural
In the BUndnerschiefer of the Avers area and overlying the Adula basement nappe (Fig. 2A, B), ultrabasic rocks such as prasinites, gabbros, serpentinites and glaucophane schists
sedimentation separated the north- the south-Penninic realm, indicating
are intercalated in the sediments, indicating volcanic activity
1986).
in the
during basin subsidence (Oberhansli More complete ophiolitic sequencesa re
1977, 1978, only known overlying south-Penninic units (Dietrich,
thus seems that the north-Penninic basement thinned continental crust.
1967). It represents
As regards the evolution of the presumed m些 里ennin丝 realm, in the Schams nappes the Middle Triassic carbonates
c o n t a i n s ma l l l a y e r s o f a l k a l i c a s h - f a l l t u f f s ( S t r e i f f e t a l . ,
1976), comparable to similar rocks in the Western Alps, which are now being linked to growth faults of Liassic age.In Middle Jurassic to Early Cretaceous times, the slowly subsiding Schams and Falknis platforms were bordered to the South by a zone of steep scarps, indicating tectonic activity by extension and/or strike-slip movements. The angles of unconformities at the base of the scarp breccias
are up to 4o 。
In the Western and Central Alps, the Briangonnais, the M6dianes- and the Klippen-units are considered as remnants of a Mesozoic structural high or "sweirl 60 to 100 km wide,
south of the north-Penninic basin (Fig. 1C, Boillot et al., 1984A). Nonetheless, it is still possible that these mid- Penninic units may also include the Schams, Falknis and Sulzfluh nappes. For the Falknis nappe, a southern (Austro-
alpine) origin has recently been re-stated (Gruner, 1981). However, this hypothesis is not consistent with the tectonic DOSition of this unit. New investigations by St. Schmid and colleagues inaicate tne possiDiiity oi an eastern origin oi ine Schams nappes with respect to their actual position.
These two alternative paleogeographic solutions are illus- trated in Figure 2B. In the hypothesis of an eastern origin of the Schams nappes (Fig. 213i), the mid-Penninic high was a laterally fragmented ridge, and the north-Penninic basin Dassed in Dlaces continuously by crustal thinning into the south-Penninic basin ksee also weissert ana ijernouin, iu6o).
In the hypothesis of a mid-Penninic origin of the Schams nappes (Fig. 213ii), a crustal block with moderate crustal stretching was intercalated between the stretched north-
discontinuous stretching, most probably coupled with strike-slip tectonics. Alternatively, the structural highs formed rather isolated islands, and a continuous transition from one deep basin to the other and from continental crust to oceanic crust may have existed in places. In any case the
margin is not a simple Atlantic type stretched margin. Crustal stretching in the Penninic basins was thus fairly discontinuous, with areas of almost normal crustal thickness in the mid-Penninic structural highs, and strong extensional tectonics in the north-Penninic basin. Continental margin formation was, therefore, quite different from the simple stretching model as published for several Atlantic margins and the southern margin of the Alpine Tethys. This may be due to sinistral strike-slip movements linked to the opening
o f t h e Te t h y s , a s p o s t u l a t e d b y L e mo i n e《 些 B o i l l o t e t a l . ,
1984), corresponding to a large scale strike-slip between the African and the European continents.Acknowledgements
Research on the evolution of the northern margin of the Alpine Tethys is sponsored by the Swiss National Science Foundation projects 2.107-0.86, 4.901-0.85.20, 4.897-0.85.20
and 2.155"0.86.
H. Funk teaches sedimentary petrography at the Swiss Federal Institute of Technology (ETH,
Sonneggstrasse 5, CH- 8092 ZUrich,
Switzerland). He is known for his research work on the stratigraphy,
sedimentology and paleogeography of the Helvetic platform in Eastern
Switzerland.
Penninic basin basin. In this
nental margin
a rea ca se
and the partly
the extension
of the TethysW a S
oceanic south-Penninic
of the European conti- very discontinuous with alternating areas
blocks of almost
Penninic highs).
of strong thinning (Priittigau basin)and no thinning of the continental crust (mid-
R. Oberhhinsli teaches magmatic rocks and ore deposits at the Mineralogical Institute of Bern (CH 3012 Bern, Switzerland). His main activity concerns the geochemistry of volcanics and the geology and geochemistry of high-pressure metamorphic rocks in the Alps.
Conclusions
On the European continent in the north, the intracontinental Jura basin and part of the Swiss Plateau were slowly sub- siding from Triassic to Late Jurassic time. However, as no extensional features are known in this basin, subsidence was most probably controlled by post-Hercynian thermal relax- ation, though this is not yet proven by paleogeothermic data.
To the south of the Plateau, the Helvetic realm, which was a 60 km wide European shelf of the Tethys, subsided by moderate crustal stretching (less than 10%), from Early Jurassic to Early Cretaceous time, as indicated by the
A. Pfiffner (Geological Institute,
Universit6 de Neuch6tel, 11 rue Emile-Argand, CH-2000 Neuch6tel, Switzerland) is coordinator of the eastern geotraverse of the Swiss NFP 20 project, aimed at exploring the deep structure of the Alps. His main research is focussed on rock deformation and Alpine tectonics.
EPISODES, Vol. 10, No. 2, June 1987 105